Lincompex systems have generally been known since the early 1960s. These systems were originally implemented with analog technology but now may be implemented digitally such as taught in U.S. Pat. No. 5,271,499 to Leveque, the invention of the present application. U.S. Pat. No. 4,271,499 is incorporated herein by reference.
The basic principles of a Lincompex system as disclosed in the '499 patent will now be described with reference to FIGS. 1(a) and 1(b). Incoming signals are applied to an input terminal 20 and are split into speech signal and control paths 21a and 21b respectively. In the control path 21b, the envelope of the input signal level is detected by an envelope detector 26 and a signal is generated by detector 26 proportional to this detected level. A compressor circuit 22 in the speech path uses the signal from the envelope detector to adjust its gain according to the detected signal level so as to output signals of substantially constant and compressed amplitude. In general implementations, the signal is a voice signal so that the gain of the compressor is adjusted at a syllabic rate. The control path signal is then converted to a logarithmic signal in a logarithmic amplification circuit 28 to compensate for the voltage to decibel conversion which occurs when compressing a voltage signal representing a logarithmic voice signal. The logarithmic signal is applied to a voltage controlled oscillator 30 which produces an output frequency related to the amount of speech signal compression conducted upon the corresponding syllable in the compressor circuit 22. The compressed speech signal and the control frequency signal are then combined in a summer 32, amplified, and input to a transmitter 34 for transmission over a transmission medium 36. The envelope detector 26, logarithmic amplifier 28, and the voltage controlled oscillator 30 form a control tone generator 24.
At the receiving end, a modulated signal is input to a receiver 38 after being transmitted over the transmission medium 36a, and is split back into speech and control signal paths 39a and 39b respectively. The control signal frequency is isolated from the demodulated signal by a band pass filter 46 and the frequency thereof is detected by a frequency demodulator 48. This frequency signal is then passed through a logarithmic to linear network or anti-log amplifier 50 to reattain the speech envelope level signal. The compressed data signal from receiver 38 is passed through a low-pass filter 40 to remove high frequency components. The envelope level signal is then applied to an expander circuit 42 in the speech path which amplifies the compressed signal, back to the original amplitude input signal.
One unresolved problem relating to Lincompex use in a communications network arises when the receiving station demodulator circuitry is in an operative state but no signals are being transmitted by the transmitter. In such a circumstance, the demodulator has only a noise input. Referring to FIG. 1(b), during normal speech transmission, the compressed speech frequency band and the control tone frequency band are as indicated in FIG. 3(a) and are received at receiver 38 of FIG. 1(b) as indicated at (a). The frequency band of the envelope of the original speech signal is shown in FIG. 3(b), while the restored voice signal frequency band is shown in FIG. 3(c). As is apparent, the control tone frequency is used to expand the compressed speech signal to the restored voice parameters as shown in FIG. 3(c) which are output from the expander circuit 42. Reference (b) indicates where the envelope signal, represented by the control tone signal, appears in the control tone conversion circuit 44. With only noise present at the input, as illustrated in FIG. 4(a), random noise responses with a frequency characteristic shown in FIG. 4(b) will be expanded as shown in FIG. 4(c) according to the fluctuations in the noise input as shown in FIG. 4(a). The designations (a), (b) and (c) of FIG. 1(b) apply also to FIGS. 4(a)-4(c). Such random fluctuations of noise at the demodulator are very disconcerting to an operator and are thus undesirable.